1. Field of the Invention
The present invention relates generally to electrical papers suitable for use in capacitors which are made from synthetic pulp fibers produced by the flash-spun method. More particularly, this invention pertains to capacitor grade dielectric tissues and electrolytic capacitor paper manufactured on a papermaking machine from flash-spun synthetic pulp fibers.
2. Description of the Prior Art
Commercially available capacitors are basically sealed units containing two metal electrodes connected to external electrical terminals, wherein the electrodes are separated by a single sheet or multiple layers of either plastic film, capacitor paper, or a film-paper combination which is usually interwound convolutely into a roll. Capacitors are usually impregnated under vacuum with a dielectric fluid prior to sealing the units. The dielectric fluid impregnant reduces the possibility of corona discharges caused by gas ionization within the capacitor and may be used to increase the capacitance of the unit.
The predominant material previously used as dielectrics or separators in commercial capacitors has been pure paper fabricated generally from wood pulp on a papermaking machine. This paper has a relatively high degree of physical uniformity and must meet rigid chemical, physical and electrical requirements as set forth by the American Society for Testing and Materials (ASTM). Papers suitable for use as separators in electrolytic capacitors are described in ASTM Specification D 2753. The standard requirements for tissues used as dielectrics in capacitors are set forth in ASTM Specification D 1930. For instance, these capacitor grade tissues must be made solely from unbleached sulfate pulp of coniferous origin in accordance with the well-known "kraft" pulping process and can not contain any coloring agents or other additives.
In electrolytic capacitors the separator paper acts as a mechanical element and contributes to dielectric strength, particularly in the higher voltage range (450 volts and higher). The principal dielectric in this type of capacitor is a very thin coating of oxide on a sheet of metal foil which serves as one of the electrodes, and the fluid impregnant or electrolyte functions as a conductor to enhance current flow. In another type of capacitor, the electrostatic type, the tissues serve primarily as dielectrics, but also function as insulators between the electrodes.
Papers meeting the above specifications have the disadvantage of retaining moisture which results from the papermaking process. It is the necessary removal of this moisture by various drying methods that proved to be both time and energy consuming, resulting in increased manufacturing costs. Also, even under the best manufacturing conditions, these papers may contain minute conductive particles within the limits of the specified requirements which can create leakage paths between the electrodes and may result in shorting out the capacitor. While the use of doubled sheets of paper will generally insure against such current leakage, this further adds to the cost of capacitors.
In recent years, dielectric paper has been gradually replaced in capacitors by synthetic plastic films of polypropylene, polyethylene, and polyesters, for example. While these synthetic films do not require the costly drying step required for capacitor grade paper, they have the disadvantage of not being able to absorb dielectric fluid. Since a dielectric fluid can greatly lengthen the useful life of capacitor units and significantly increase their electrical properties such as capacitance, dielectric strength and dielectric constant, impregnation with this fluid is highly desirable. Moreover, since non-absorbant films do not significantly benefit from impregnation, capacitance is primarily determined by the dielectric constant of the film. Therefore, some capacitors utilize a sheet of paper in combination with a sheet of film, wherein the absorbent paper serves as a wick to hold the dielectric impregnant.
In addition to being non-absorbant, synthetic films allow gas pockets to form between sheets of film when wound tightly together. This can cause corona discharges within the capacitor, creating heat, unnecessary energy loss, and possible dielectric failure. Moreover, films contain weak spots generally caused by resin impurities which may require using multiple sheets of film between electrodes, particularly in high voltage capacitors.
Dielectric paper for use between capacitors electrodes consisting of wood pulp fibers and 2-10% of relatively large-diameter rayon or polyvinyl formal fibers as bulking agents is disclosed in U.S. Pat. No. 3,385,752 to Selke et al. However, the disclosed finished sheet is predominantly of wood origin and would therefore be subject to the same costly moisture removal step associated with the prior art papermaking procedure. Also, the density of this reference paper is relatively high and its porosity is fairly low, making impregnation with a dielectric fluid less efficient.
Another such paper for electrolytic capacitors is described in U.S. Pat. No. 4,914,548 to Kubo et al. This electrolytic paper is made of cellulose fibers provided with organic substituents induced by chemical treatment to improve its swelling degree. While the reference paper has an increased absorption rate and decreased density, the chemical treatment step significantly adds to the manufacturing cost of the paper and the induced substituents may adversely effect the physical and electrical properties of the treated paper.
The prior art further discloses cellulosic products which may contain synthetic fibers for various electrical applications. For instance, U.S. Pat. No. 4,196,044 mentions the use of blends of natural and synthetic fibers in making insulating paper for transformers and U.S. Pat. No. 4,833,011 to Horimoto discloses synthetic pulp fibers of a thermoplastic resin and a high molecular weight surface active agent which may be used for the production of electrical paper. In addition, U.S. Pat. Nos. 4,595,457 and 4,752,355 relate to oil impregnatable pressboards having electrical insulating properties which are made of aromatic polyamide fibers. However, none of the electrical papers described in the present patent literature conform to the critical chemical and physical requirements of industry specifications for capacitor grade tissues and electrolytic capacitor papers. Also, most of the prior art electrical papers contain ingredients, such as coloring agents and other additives, or surface active agents as in the Horimoto patent, which are strictly prohibited by ASTM standards from capacitor grade papers suitable for use as dielectrics or separators.